1. Field of the Invention
The present invention concerns a variable geometry turbine, in particular for a turbocharger for a supercharged internal combustion engine, and said turbocharger and engine.
2. Prior Art
Known variable geometry turbines (VGT) have a drive fluid inlet in form of a scroll surrounding the turbine rotor, and a vaned annular nozzle located between said inlet scroll and the turbine rotor. On VGT's of the “moving wall” type, the nozzle gap is axially adjustable to control the power of the turbine and, in case of VGT turbochargers for supercharged internal combustion engines, the back pressure at the exhaust manifold of the engine (this is particularly useful when used as exhaust brake).
FIG. 1 shows a typical “moving wall” VGT. The figure represents a longitudinal section, according to a plane containing the axis 10 of the turbine rotor 4 (shown not sectioned). The fluid inlet scroll is designated by 1. The annular nozzle gap allowing the fluid flowing from the inlet scroll to the turbine rotor is designated by 3, a vane of the nozzle by 7. The vanes are fixed to the axially adjustable ring 5, apt to translate in the direction of the arrow A. The movement in one direction reduces the nozzle gap, the movement in the opposite direction increases it. When the adjustable ring is moved in the closing direction, the vane grid is received into the annular hollow 11 provided in the turbine housing 2; a pierced shroud 8 with slots corresponding to the shape of the vane grid can be foreseen to prevent the fluid flow bypassing the nozzle gap. Alternatively, a configuration can be provided featuring a fixed vane grid on the wall opposite to the axially adjustable ring, and the axially adjustable ring featuring slots to accommodate the vanes. The adjustable ring can be configured as an annular piston, being housed inside an annular chamber 12, able to move out and to extend into the nozzle gap; sealing means 13, such as an outer and an inner sealing ring, are placed between the adjustable ring and the chamber walls. An actuating system (not shown) is provided to control the axial position of the adjustable ring according to the requirements. The actuator can be pneumatic, hydraulic, or electric, possibly comprising reset springs, and may be placed inside or external to the VGT housings. It can act, for example, through rods (not shown) extending along the direction of movement of the axially adjustable ring, said rods attached to the ring 5 on the side facing the annular chamber 12. The rods (or any similar guide system) may prevent rotation of the adjustable ring around axis 10, which would be caused by fluid forces against the inclined vanes. The actuator must hold the resetting force exerted by the fluid pressure onto the axially adjustable ring, which can be of considerable magnitude. In order to reduce the resetting load onto actuator and actuating mechanism, balance holes 6 are commonly provided on the axial wall of the adjustable ring, normally one hole for each fluid passage between two consecutive vanes, as shown in FIG. 2, in order to balance the pressure between the nozzle gap 3 and the chamber 12.
On exhaust gas driven turbines, the size of the balance holes must be matched to allow proper transmission of the exhaust pressure waves generated by reciprocating engines, such as standard internal combustion engines. The pressure waves otherwise could generate vibration wear on the whole VGT actuating mechanism, and on other members such as seals and bushings. Moreover, the pressure waves can provoke significant oscillation of the actuating mechanism, at least with certain types of actuators, in particular pneumatic and electric ones. To keep vibration and oscillation on an acceptable level, the size of the balance holes has to be relatively large, their diameter may reach up to 90% of the fluid passage width. This includes the disadvantage that the balance holes cause considerable disturbance to the fluid flow through the nozzle gap. Reducing the nozzle gap, respectively narrowing the fluid passage between consecutive vanes, increases the interference of the holes with the fluid flow. At very narrow nozzle gaps, the remaining flow area in the nozzle gap becomes less than the total area of the balance hole array. In such condition, the balance holes represent significant sinks for the flow, leading to a expansion of the fluid flow into the holes, in turn the downstream edges of the balance holes become fluid-dynamical significant obstacles.
On known pressure balance arrays, the balance holes are machined or laser cut, and have (in fluid-dynamical view) relatively sharp edges, although a small countersink may be provided in order to remove burrs. The downstream portion of such balance hole edges therefore can provoke flow separation, leading to an undesirable pressure drop in the flow passage downstream the hole, and thus to a reduction in the resetting force exerted onto the adjustable ring. This effect becomes most obvious at small nozzle gaps and choke flow condition, where sonic speed is reached in the nozzle gap. In said condition, the resultant resetting force acting onto the axially adjustable ring can drop sharply, and can end up in reversed force direction.
In FIG. 3 the resetting force F (in ordinate) typically acting on the adjustable ring, and from there on the whole actuating mechanism, is qualitatively represented as a function of the nozzle gap L (in abscissa). As the nozzle gap decreases, a gradual increase of the resetting force is observed for almost the whole adjustable range, while at a narrow nozzle gap the force collapses due to the interference of the holes with the fluid flow. Safe control of the variable geometry nozzle at such small nozzle gaps thus becomes impossible, limiting the minimum admissible nozzle gap to values much higher than those that would be desirable under particular operation conditions. Generally speaking the control of known systems is unsatisfactory at small nozzle gaps. The turbo brake power (engine brake power) of VGT-supercharged vehicle engines is therefore limited, as well as engine response in transient operation conditions.